Printed circuit boards (PCB) are a fundamental element in modern electronics. A PCB includes a substrate on which conductive traces are positioned. At locations on the conductive traces, connection mounting pads are exposed to allow attachment of electronic devices, such as integrated circuits. A top layer of the PCB is commonly a solder mask. This mask is a thin layer of material that is resistant to wetting by the solder. The solder mask exposes the connection mounting pads through holes in the solder mask. The conductive trace then extends above the level of the solder mask.
To attach an electronic device to the circuit board the electronic device must be positioned such that a conductive means (ball, pin, lead, or other terminal) may be connected to the mounting pad. This is generally done by soldering the terminal on the integrated circuit to the mounting pad.
Commonly integrated circuits will be soldered onto the mounting pads on a PCB either by hand or by an automated machine. In either case care must be taken. The integrated circuit or other electronic device must be properly aligned with the correct contacts to function properly. In addition, if the solder is not properly applied, it can cross between mounting pads, creating electrical shorts that would adversely effect the functioning of the product.
In one application, printed circuit boards are used in prototyping. A company designing electronics must first create a functional prototype. Contemporary hardware design engineers generally use either a prototype printed circuit board or manufacture a custom PCB for the initial prototype. However the cost of producing a custom PCB is very high. With every iteration in design, a new custom PCB must be produced. This is both expensive and time consuming.
In contrast a prototyping PCB can save both the time and the cost of manufacturing customized boards. Changes can be made quickly and comparatively inexpensively. However use of prototyping boards also has drawbacks. The prototyping board must support the surface mount components. These mounts will generally need to be designed into the board. A high level of manual dexterity is needed to successfully hand solder the small components onto a board.
Two problems impede a user'is ability to hand solder surface mount components onto a board. First, the user must place and hold the component in place. Many of the component devices are quite small, and must be precisely positioned on pitches having small traces. The trend has been for increasingly small circuits and electronic components, exasperating this problem. Second, the user after positioning the component must be able to solder the component into a specific location without creating short circuits to traces.
In the use of mechanized systems, which is less common for prototyping and more common for manufacturing, some of the limitations of hand soldering are reduced. However, new problems arise. For example, in some manufacturing processes, the integrated circuit or other electronic device is positioned on a board and then the board with a surface positioned electronic device are moved into an oven. In the oven applied heat melts the solder, attaching the electronic device to the board. The terminals of the electronic device rest on top of the mounting pads as the devices are positioned. As integrated circuits and other electronic components become smaller, the risk of displacement and misalignment increases. First, the accuracy of automated positioning systems may not be sufficient for accurate placement of very small devices having terminals with very small pitch width. Second, the convection currents in ovens may be sufficiently strong to displace very small components.
Ideally, a soldering solution would be adaptable to both aid in prototyping and hand soldering by reducing time and skill level required, and provide a solution for manufacturing processes requiring attachment of components to boards.
Prior references address some of the issues relating to soldering leads onto mounting pads. For example, U.S. Pat. No. 5,386,087 discloses a printed circuit board having a circuit pattern covered with a solder mask layer. This solder mask layer has a U-shaped recess at the area of the solder pad. During soldering molten solder is retained in the U-shaped recess, reducing the risk of solder bridging the area between two terminals However the mask does not extend to all sides of the mounting pad, and escape of some of the soldering material is possible. In addition the relatively short length of the surrounded area may make placement and soldering more difficult.
It is an object to provide a printed circuit board that makes both the positioning of an electronic component on a PCB and attachment of the component to the PCB more reliable and less error prone. It is a further object to attain these advantages both in a manual operation and in mechanized processes.